1. Field of the Invention
The present invention relates to a zoom lens system and a camera including the zoom lens system.
2. Description of the Related Art
In recent years, image pickup apparatuses including a solid-state image pickup element, such as video cameras, digital still cameras, broadcast cameras, and security cameras, and silver-halide film cameras have been functionally advanced. In addition, the sizes of the image pickup apparatuses have been decreased.
Accordingly, imaging optical systems (zoom lenses) used in such image pickup apparatuses are required to have a short lens length, a small size, a high zoom ratio, and a high resolution.
To satisfy these requirements, a zoom lens of a rear focusing type has been developed. A zoom lens of a rear focusing type includes a first lens unit disposed closest to an object and other lens units. By moving the other lens units, focusing is performed.
One type of a zoom lens of a rear focusing type is a four-unit zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power in this order from an object side to an image side.
Among these zoom lenses, some zoom lenses move the second lens unit so as to vary magnification. In addition, the zoom lenses move the fourth lens unit so as to compensate for an image plane variation caused by the magnification (refer to, for example, Japanese Patent Laid-Open Nos. 7-270684 (corresponding to U.S. Pat. No. 5,963,378), 11-305124 (corresponding to U.S. Pat. No. 6,166,864), and 2005-215518 (corresponding to U.S. Pat. No. 7,193,787)). In such zoom lenses, only two lens units are movable. Accordingly, the zoom lenses are achieved by using a relatively simple driving mechanism.
In contrast, some zoom lenses move all of the four lens units when zooming is performed (refer to, for example, Japanese Patent Laid-Open Nos. 2000-347102 (corresponding to U.S. Pat. No. 6,867,925) and 2003-315676 (corresponding to U.S. Pat. No. 6,975,461)).
Such zoom lenses can control the sensitivity to the eccentricity of each of the lens units. Accordingly, the zoom lenses are suitable for image pickup apparatuses of a collapsible mount type in which the lens units are collapsed when the image pickup apparatuses are not in use so that the image pickup apparatuses are easy to carry.
On the other hand, in recent years, light transmissive ceramics have been developed, and therefore, lens apparatuses using such light transmissive ceramics for the optical material thereof have been developed.
Light transmissive ceramics have an index of refraction higher than that of optical glass. In addition, light transmissive ceramics have excellent hardness and strength. By using such properties of light transmissive ceramics, an optical system having a reduced thickness of an entire lens system has been developed (refer to, for example, Japanese Patent Laid-Open No. 2006-84887 (corresponding to U.S. Pat. No. 2006/0062569)).
In recent years, a zoom lens used for image pickup apparatuses are strongly required to have a high zoom ratio and a small size of the entire lens system.
In general, by reducing the number of lenses of a zoom lens while increasing the index of refraction of each of the lens units, the size of the zoom lens can be reduced.
However, for such a zoom lens, the thickness of a lens tends to increase. Accordingly, the lens system is not satisfactorily reduced. In addition, it is difficult to correct a variety of aberrations of the lenses.
To obtain a high zoom ratio and a small size for the entire lens system, it is important to determine the zoom type, the index of refraction of each of the lens units, and a combination of lens elements in each of the lens units in an optimal manner.
In particular, for the above-described zoom lenses of a rear focusing type, it is important to optimally design the structure of the second lens unit used for primarily varying magnification.
For example, for lenses in the second lens unit, a material that can reduce a variety of aberrations including a chromatic aberration at any zoom position needs to be selected in consideration of the indices of refraction and the Abbe numbers.
The lens apparatus described in Japanese Patent Laid-Open No. 2006-84887 includes a cemented lens having a positive lens element and a negative lens element bonded to each other. A material of the negative lens element is a light transmissive ceramic. In this way, the thickness of the cemented lens is reduced, and therefore, the size of the lens apparatus is reduced.
Ceramics have an index of refraction, hardness, and bending strength higher than those of glass. In Japanese Patent Laid-Open No. 2006-84887, using these properties of ceramic, the size of the lens apparatus can be advantageously reduced by an amount of reduction in the thickness of one negative lens element.
Consider a graph having the ordinate representing an index of refraction increasing upward and the abscissa representing the Abbe number increasing towards the left (hereinafter referred to as an “nd-νd diagram”). In general, when glass is mapped on the graph, the property of the glass distributes along one of several straight lines.
In general, as the index of optical glass increases, the Abbe number of the optical glass decreases and the dispersion increases.
In the nd-νd diagram, some ceramics, single-crystalline oxide materials, and polycrystalline oxide materials having a high light transmission in the visible light range have a relationship between the index of refraction and the Abbe number different from the relationship of a normal optical glass.
That is, these materials have an index of refraction higher than that of optical glass having the same Abbe number.
When a ceramic having such property is used for an optical material, aberrations can be easily corrected and the size of the entire lens system can be easily reduced.
However, even if a lens made of a ceramic is simply used for a zoom lens, it is difficult to obtain a high optical performance for the entire zoom range.
To increase a zoom ratio and reduce the size of the entire lens system, it is important to determine the zoom type, the index of refraction of each of the lens units, and a combination of lens elements in each of the lens units in an optimal manner.